1. Field of the Invention
The present invention relates to a constant-voltage power supply circuit which controls a power supply voltage to be applied to a load to a set constant voltage, and more particularly, it relates to an improvement in a current limiting protection circuit for limiting flowing of an overcurrent to a transistor which controls the power supply voltage at the constant voltage and protecting the same.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional constant-voltage power supply circuit. In the drawing, a power source is applied to an input terminal 1. A PNP transistor 3 for controlling the power supply voltage has an emitter connected to the input terminal 1, a collector connected to an output terminal 19 and a base connected to the emitter through a resistor 2 as well as grounded through a constant-current circuit 4 and a reference voltage source 5 in a series manner. PNP transistors 6 and 7 have their bases and emitters connected in common with each other. The common-connected emitters are connected to a junction between the base of the PNP transistor 3 and the resistor 2 and the constant-current circuit 4. The base and the collector of the PNP transistor 6 are connected in common. The emitters of NPN transistors 8 and 9 are connected in common, and the junction therebetween is grounded through an emitter resistance 10. The NPN transistors 8, 9 and the emitter resistor 10 form an error amplification circuit 11. The collector of the NPN transistor 8 is connected to the collector of the PNP transistor 6. The base of the NPN transistor 8 is connected to the junction between the constant-current circuit 4 and the reference voltage source 5. The collector of the NPN transistor 9 is connected to the collector of the PNP transistor 7. The base of the NPN transistor 9 is connected to an output regulating terminal 20 which is a junction between resistors 16 and 17 connected in series between the output terminal 19 and a ground 18. The voltage applied to a load 21 is detected by the NPN transistor 9 in the error amplification circuit 11 and the PNP transistor 7, so that the base bias of the control PNP transistor 3 is changed, thereby to stabilize the voltage applied to the load 21.
The control NPN transistors 12 and 13 are interconnected in a Darlington connection manner, and the common-connected collectors thereof are connected to the junction between the base of the control PNP transistor 3, the resistor 2 and the constant-current circuit 4. The base of the NPN transistor 12 is connected to the collector of the PNP transistor 7, while the emitter of the NPN transistor 13 is grounded through a resistor 14 which detects the current flowing to the NPN transistor 13. The collector of a current limitation detecting NPN transistor 15 is connected to the collector of the PNP transistor 7, and the base and the emitter of the NPN transistor 15 are connected to both ends of the resistor 14. The load 21 is connected between the output terminal 19 and the ground 18.
When, in the aforementioned construction, a constant and stabilized voltage V.sub.ref is fed to the base of one NPN transistor 8 in the error amplification circuit 11 while the base of the other NPN transistor 9 is supplied with a voltage divided by the resistors 16 and 17 from an output voltage V.sub.0 at both ends of the load 21, the error amplification circuit 11 compares the two voltages so as to control the control PNP transistor 3 such that the difference therebetween becomes zero, thereby maintaining the output voltage constant. Assuming here that the base-to-emitter voltage V.sub.BE of the NPN transistors 8 and 9 are equal and the resistance values of the resistors 16 and 17 are respectively represented by R.sub.16 and R.sub.17 while the voltage value of the reference voltage source 5 is represented by V.sub.ref, the output voltage V.sub.0 can be set as follows: ##EQU1## The change in the output voltage V.sub.0 is divided by the resistors 16 and 17 and is fed to the base of the NPN transistor 9 in the error amplification circuit 11, to be compared with the voltage V.sub.ref at the reference voltage source 5, and the change is thus detected. For example, when the output voltage V.sub.0 is changed to a higher value, the changed voltage is divided by the resistors 16 and 17, and thereby the base potential of the NPN transistor 9 in the error amplification circuit 11 is increased, leading to increase in the collector current of the NPN transistor 9. Thus, the base current to the Darlington-connected control NPN transistors 12 and 13 is reduced, leading to decrease in the collector current of the Darlington-connected control transistors 12 and 13, and the base current of the control PNP transistor 3 is reduced. By virtue of this, the collector potential of the PNP transistor 3, i.e., the output voltage V.sub.0 is reduced. When, to the contrary, the output voltage V.sub.0 is lowered, the error amplifiction circuit 11 operates the other way to the above to raise the output voltage V.sub.0. Thus, the Darlington-connected control transistors 12 and 13 and the control PNP transistor 3 are controlled such that the base potential of the NPN transistor 9 is made equal to the voltage V.sub.ref at the reference voltage source 5, thereby maintaining the output voltage constant against change in the input voltage and load change.
The resistor 14 for detecting the current is connected between the base and the emitters of the current limitation detecting NPN transistor 15. When the voltage produced by the current flowing to the resistor 14 exceeds a predetermined value, the NPN transistor 15 is turned on, thereby controlling the current flowing to the NPN transistors 12 and 13 for controlling the base bias of the transistor 3. Thus, current limiting protection is applied so that the transistor 3 is not subjected to an overcurrent exceeding a predetermined set value.
In the aforementioned current limiting protective circuit, the transistor 3 for controlling the power supply voltage to the load at constant can be prevented from being subjected to an overcurrent. However, under an extraordinary condition such that the load is short-circuited, the conventional detection circuit consisting of the transistor 15 and the resistor 14 for preventing flowing of an overcurrent to the transistor 3 may not sufficiently protect the transistor 3. In other words, even in a case the load is short-circuited, the detection circuit merely performs the current limiting protective operation identical to the above overcurrent limiting operation. Therefore, in this case, the collector current of the transistor 13 is fed as the base current of the control PNP transistor 3, whose collector current in turn flows being amplified to a value multiplied by d.c. forward current transfer ratio h.sub.FE (=collector current/base current) of the control PNP transistor 3. Thus, a remarkably large electric power is applied to the control PNP transistor 3, which in the result is damaged.